This invention relates to the preparation of phosphoramidates, and more particularly to their preparation from nitrogen bases and diaryl chlorophosphates.
The use of sterically hindered phosphoramidates such as N,N'-bis[di-(2,6-xylenoxy)phosphinyl]piperazine (hereinafter sometimes "XPP") as flame retardant additives for synthetic resins, especially thermoplastic resins such as polycarbonates, ABS resins and blends thereof, has been discovered to have particular advantages including improved high temperature stability of the resulting blends. Reference is made, for example, to U.S. Pat. No. 5,973,041 and to copending, commonly owned applications Ser. Nos. 09/235,679 and 09/364,915.
XPP and analogous compounds may be conveniently prepared by the reaction of a diaryl chlorophosphate, such as di-(2,6-xylyl) chlorophosphate, with a heterocyclic compound containing two basic N--H groups, such as piperazine. According to the prior art as illustrated by Talley, J. Chem. Eng. Data, 33, 221-222 (1983), this reaction may be conducted in chloroform as solvent, in the presence of triethylamine as an acid acceptor. The triethylamine is employed in stoichiometric amount or in excess, and reacts with the by-product hydrogen chloride to drive the reaction to completion.
The Talley paper describes the preparation of a number of analogous compounds including those derived from such nitrogen compounds as benzylamine, cyclohexylamine, aniline, ethylenediamine and p-phenylenediamine as well as piperazine. Reported yields were as high as 90% for the reaction with aniline, and as low as 61% for the reaction with p-phenylenediamine. Piperazine afforded XPP in a yield of only 68%, one of the lowest reported.
If the use of XPP as a flame retardant additive is to be commercially feasible, it is necessary to improve its yield by a significant amount. Also, it is desirable to minimize use on a commercial scale of the relatively toxic solvent chloroform, as well as other solvents which can add complexity to the reaction by reason of the need for isolation of product from the solvent, as well as disposal or recycle of solvent and of extraneous acid acceptors such as triethylamine.
It is of interest, therefore, to develop high-yield methods for the preparation of XPP and analogous compounds with a minimum of unit operations.